var banner = new Array();

banner.push('http://img510.imageshack.us/img510/6237/bann1u.png');
banner.push('http://img502.imageshack.us/img502/7845/bann3.png');
banner.push('http://img341.imageshack.us/img341/8306/bann2.png');

document.getElementById('i_logo').src= banner[Math.floor(Math.random()*banner.length)];